1. Field of the Invention
The invention relates to a method and apparatus for precisely, in both multishot (discrete) and continuous borehole surveys, mapping boreholes during Measurement-While-Drilling (xe2x80x9cMWDxe2x80x9d) and WireLine (xe2x80x9cWLxe2x80x9d) logging operations. More particularly, the invention relates to a strapdown gyrocompassing and inertial navigation system vibrating mass,. Coriolis effect gyroscopes for determining the precise path of deep, small diameter boreholes.
2. Description of the Related Art
Borehole survey systems used for geological surveying and drilling of oil and gas wells generally map or plot the path of a borehole by determining borehole azimuth and borehole inclination at various points along the borehole. xe2x80x9cAzimuthxe2x80x9d may be considered, for present purposes, to be the directional heading relative to a reference coordinate, such as north. xe2x80x9cBorehole inclinationxe2x80x9d may be considered, also for present purposes, the deviation from the vertical.
For example; in one type of known system, a tool or probe contains several magnetometers for indicating azimuth and several accelerometers for indicating inclination. The probe is suspended by a cable, and then raised and lowered through the borehole. In such a system the directional coordinates of the probe are determined at several points along the borehole. When sufficient measurements at discrete points along the borehole are made, they are plotted to map the borehole. The map can be determined relative to a desired coordinate system. This type of system has become known as a WireLine (xe2x80x9cWLxe2x80x9d) survey system.
A second type of known system is utilized during what has become known as Measurement-While-Drilling (xe2x80x9cMWDxe2x80x9d) operations. MWD equipment traditionally includes magnetometers and accelerometers disposed on each of three orthogonal axes of a tool, or probe, in a drill string assembly. Such a probe has typically been part of a special drill collar placed a relatively short distance above a drilling bit. The drilling bit bores the earth formation as the drill string is turned in the wellbore by a rotary table of a drilling rig at the surface. The drill string""s rotation is periodically stopped so that the probe may generate magnetometer data regarding the earth""s magnetic field and accelerometer data regarding the earth""s gravitational field. The data is generated with respect to the orthogonal axes of the probe. The magnetometer data and the accelerometer data are then used to determine the heading of the well.
These techniques suffer from the fact that the earth""s magnetic field varies with time. Also, the measurable magnetic field is affected by structures containing iron or magnetic ores near the probe. Even the casing supporting the wellbore can vary the measurable magnetic field. Such variation leads to errors and uncertainty in the determination of the well heading by undesirably influencing the data.
Various considerations have brought about an ever increasing need for more precise and compact borehole surveying techniques. For example, modern gas and oil drilling techniques have brought about smaller diameter boreholes and often require that wells be closely spaced. Additionally, it is not unusual for a number of wells to be drilled toward different geological targets from a single wellhead or drilling platform. Further, depletion of relatively large deposits has made it necessary to drill deeper and to access smaller target formations. Even further, in the event of a deep, high-pressure blowout, precise knowledge of the borehole path is required so that a relief well can be properly drilled to intercept the blowout well.
One proposal for providing a small diameter probe for a borehole survey system and for yielding more accurate. measurements applies inertial navigation techniques. Generally speaking, inertial navigation techniques utilize a set of accelerometers and a set of gyroscopes. The accelerometers supply signals representing acceleration of the instrumentation package along the three axes of a Cartesian coordinate system. The gyroscopes supply signals representative of the angular rate at which the instrumentation package is rotating relative to that same Cartesian coordinate system. Magnetic field variations can theoretically be eliminated by adding gyroscopes to each of the orthogonal axes of the probe. The heading of the probe can then be determined from accelerometer data from each of such axes and gyroscopic data from each of such axes. The accelerometer data is responsive to the gravitational field of the earth, while the gyroscopic data is responsive to the rotational velocity of the earth with respect to inertial space.
The first basic type of Inertial Navigation System (xe2x80x9cINSxe2x80x9d) is the xe2x80x9cgimballedxe2x80x9d system. In gimballed systems, the gyroscopes and accelerometers are mounted on a fully gimballed platform maintained in a predetermined rotational orientation by gyro-controlled servo systems. This arrangement effectively maintains the accelerometers in fixed relationship so that the accelerometers provide signals relative to a coordinate system substantially fixed in inertial space. Successive integration of the acceleration signals with respect to time yields signals representing the velocity and position of the instrumentation package in inertial space. However, known gimballed systems have generally been unsatisfactory because of the size of the gimbals required for the gyroscopes. Such systems do not readily withstand the shock, vibration and temperature inherently encountered in the survey of deep boreholes. In addition, gyroscope drift, precession, sensitivity to g-forces and other factors seriously affect system accuracy.
The second basic type of inertial navigation system is the xe2x80x9cstrapdownxe2x80x9d inertial navigation system. In strapdown systems, the gyroscopes and accelerometers are fixed to and rotate with the instrumentation package and, hence, with the borehole survey probe. In such a system, the accelerometers provide signals representative of the instrument package acceleration along a Cartesian coordinate system that is fixed relative to the instrumentation package. The gyroscope output signals are processed to translate the measured accelerations into a coordinate system that is fixed relative to the earth. Once translated into the earth-referenced coordinate system, the acceleration signals are integrated in the same manner as in a gimballed navigation system to provide velocity and position information. In many known strapdown systems (or hybrid strapdown configurations in which the accelerometers are gimballed relative to the longitudinal axis of the probe), the probe must be frequently stopped to correct for velocity errors that are caused by instrument drift.
One strapdown INS for surveying boreholes is disclosed in U.S. Pat. No. 4,812,977, issued Mar. 14, 1989, to Sundstrand Data Control, Inc. as the assignee of the inventor Rand H. Hulsing, II (xe2x80x9cthe ""977 patentxe2x80x9d). The ""977 patent discloses a strapdown INS for use in WL operations. This particular INS employs ring laser gyroscopes, which have proven to be unreliable for MWD operations because of their susceptibility to shock, temperature, and other drilling conditions.
A hybrid strapdown/gimballed INS for surveying boreholes is disclosed in U.S. Pat. No. 4,987,684, issued Jan. 20, 1991, to The United States of America, as the assignee of the inventors Ronald D. Andreas, et al. This INS employs two dual axis gyroscopes and three single axis accelerometers strapped to a sensor block that is gimballed. Thus, although partially strapped down, the gimbal will nevertheless necessitate undesirably large dimensions for the tool. Also, this particular INS, like that in the ""977 patent, is limited to WL operations.
The present invention is directed to resolving one or all of the problems mentioned above.
The invention is, in its various aspects, a method and apparatus useful for strapdown inertial navigation and surveying in a borehole.
In a first aspect of the invention, a method comprises maneuvering a probe including at least three vibrating mass, Coriolis effect gyroscopes in a borehole and initializing the probe""s attitude in the borehole within the probe""s frame of reference. The incremental rotation angles are determined from the gyroscopes. Three orthogonal, incremental rotation angles and three orthogonal, incremental velocities are determined for the probe within the probe""s frame of reference. The method then translates the three incremental velocities from the probe""s frame of reference into the inertial frame of reference using the three incremental rotation angles. Next, a velocity vector in a local-vertical, wander-azimuth frame of reference is determined from the translated incremental velocities. A velocity error observation is then obtained. A system error is then estimated from the velocity vector and the velocity error observation. The system error is then fed back into the inertial navigation system for use in refining the method""s analysis.
In a second aspect, the invention is a strapdown, inertial measurement unit. The inertial measurement unit includes a housing, three accelerometers, and three vibrating mass, Coriolis effect gyroscopes. The three accelerometers are mounted within the housing. The three vibrating mass, Coriolis effect gyroscopes are rigidly mounted within the housing.